We demonstrate a continuous-wave 2.1-µm laser with a new Ho:GdTaO4 crystal pumped by a 1940.3-nm Tm fiber laser at room temperature. The maximum output power of 11.2 W at 2068.39 nm was achieved, corresponding to a slope efficiency of 72.9%. Moreover, the beam quality factor was measured to be about 1.4 at the maximum output level.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
CorrectionsXiaoming Duan, Guangpeng Chen, Chuanpeng Qian, Yingjie Shen, Renqin Dou, Qingli Zhang, Linjun Li, and Tongyu Dai, "Resonantly pumped high efficiency Ho:GdTaO4 laser: erratum," Opt. Express 27, 31362-31362 (2019)
The 2.1-μm holmium (Ho) lasers based on the 5I7→5I8 transition are valuable for many technical applications such as range finder, medical surgery, water vapor profiling, wind monitoring and spectroscopy. Moreover, it is an excellent pump source of the middle infrared nonlinear optical frequency conversions (optical parametric oscillator and optical parametric amplifier etc.). Resonantly pumping technology provides weak thermal load and up-conversion loss in Ho-doped materials. As results, the resonantly pumped Ho laser can be achieved with high efficiency and high output power at room temperature . In the past twenty years, many oxide and fluoride materials were successfully applied to dope the Ho ions in order to obtain 2.1-μm laser radiations [2–11]. Among traditional Ho3+ singly doped crystals, Ho:YAG attracts most of the attention and is widely used to obtain high output power. However, they show strong pump wavelength sensitivity due to its narrow absorption bandwidth at around 1.9 µm. Moreover, thermally induced birefringence of Ho:YAG crystal significantly affects the its lasing performance under high power pumping conditions. Therefore, there are many efforts to search new laser materials.
Recently, gadolinium tantalite GdTaO4 (GTO) crystal was used as promising host for doping of rare earth. The GTO crystal has low symmetry and strong symmetrical crystal field, which is beneficial for obtaining polarized laser output and enhancing the photoluminescence efficiency. Compared with Ho:YAG, the GTO crystal has low sensitivity for thermal-optical effect. Owing to this advantage, the Nd:GTO crystal becomes a high-performance 1-µm laser material in the past few years. In 2015, the spectral properties and continuous wave (CW) 1066-nm laser performance of Nd:GTO crystal have been demonstrated . At the same year, the passively mode-locked Nd:GTO laser was reported with pulse duration of 750 ps at 1066 nm . In 2018, the CW and actively Q-switched Nd:GTO laser was presented with maximum CW output power of 1.93 W at 1066 nm and shortest pulse duration of 28 ns at pulse repletion rate of 10 kHz . However, there is less work on the 2-µm laser action of RE-doped tantalite materials up to the present . In 2018, a 0.33 W diode-pumped Tm,Ho:GdYTaO4 laser with dual-wavelength of 1949.677 nm and 2070 nm was reported.
In this paper, to the best of our knowledge, we demonstrate the 2.1-µm lasing performance of Ho-singly-doped GTO crystal for the first time. A Ho:GTO crystal with dopant concentration of 1.0 at.% was used as the gain medium. Using a FBG-locked 1940.3-nm Tm fiber laser as the pump source, the maximum output power of 11.2 W at 2068.39 nm and the slope efficiency of 72.9% were reached in end-pumped Ho:GTO laser at heatsink temperature of 15 °C. Moreover, the beam quality factor (M2) was estimated to be 1.4 at maximum output level.
2. Spectral properties
The M-type Ho:GTO crystal (space group I2/a, site symmetry C2) with dopant concentration of 1.0 at.% was grown along the c-axis by the Czochralski method at the Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. Ho:GTO chips for spectral measurements with thickness of 2 mm were cut along a-, b- and c-axis. Their end faces were optically polished. The spectral measurements were all performed at 300 K. The polarized absorption spectra in the range from 1800 nm to 2200 nm were recorded by the UV-VIS-NIR spectrophotometer (Shimadzu SolidSpec-3700, as shown in Fig. 1(a). It can be seen that there are six absorption peaks of 1874 nm, 1905 nm, 1914 nm, 1921nm, 1932 nm and 1947 nm, indicating that the pump wavelength has great flexibility for selection of Tm-doped bulk or fiber lasers. A fluorescence spectrometer (Horiba iHR550) was employed to measure the fluorescence spectra under 1920-nm Tm:YLF laser exciting. Figure 1(b) shows the normalized emission spectra of Ho:GTO crystal in the range from 1800 nm to 2200 nm. The strongest emission peak was located at 2070 nm for three polarized spectra. Moreover, there are two weaker emission peaks of 2056 nm and 2083 nm. With an OPO (Opolette 355I) laser exciting, the decay curve of 5I7 level of Ho ions was measured by a fluorescence spectrometer (Edinburgh FLSP920), as shown in Fig. 2. It has single-exponential characteristic, and the lifetime was fitted to be 6.54 ms.Figure 3(a) shows the polarized absorption cross sections of Ho:GTO crystal. It can be seen that the strongest absorption peaks are located at 1932 nm, 1947 nm and 1921 nm for E//a, E//b and E//c, respectively, corresponding to the cross sections of 0.84 × 10−20 cm2, 0.62 × 10−20 cm2 and 0.68 × 10−20 cm2. These absorption peaks are beneficial to high power Tm-laser pumping.18]. With Eq. (2), the polarized emission cross sections of Ho:GTO crystal were calculated and shown in Fig. 3(b). As a result, the maximum emission peak is located 2070 nm with cross sections of 1.01 × 10−20 cm2, 1.03 × 10−20 cm2 and 0.78 × 10−20 cm2 for E//a, E//b and E//c, respectively, compared with that of 1.16 × 10−20 cm2 at 2090 nm in Ho:YAG crystal  and 1.50 × 10−20 cm2 at 2050 nm in Ho:YLF crystal .
3. Experimental setup
Figure 4 schematically illustrates the experimental setup of resonantly-pumped Ho:GTO laser. A 30 W FBG-locked Tm fiber laser was employed as the pump source, which has central wavelength of 1940.3 nm and M2 factor of 1.3. Two lenses (f1 = 8 mm, f2 = 75 mm) were employed to collimate and focus the pump light into the Ho:GTO crystal. The 1/e2 pump radius was approximately 0.17 mm. The pump Rayleigh length zr (zr = πωp2n/λpMp2) was calculated to be about 75.9 mm inside the Ho:GTO crystal with refraction index of 2.11. The c-cut Ho:GTO crystal was used as the gain medium, which has dimensions of 4 × 4 mm2 (in cross section) and 24 mm (in length). The actual single-pass pump absorption was measured to be 83.7% under no-lasing conditions. Both end-faces of crystal were polished and antireflection coated for pump and resonant wavelength. The Ho:GTO crystal was wrapped with 0.1-mm-thickness indium foils and mounted in a cooper heatsink. The heatsink temperature was controlled by the thermoelectric cooler. A two-mirrors linear cavity was employed to investigate the output performance of Ho:GTO laser. The input mirror M1 was a plat mirror with high transmission (~94%) for pump wavelength and high reflective (~99.8%) for resonant wavelength. The output coupler M2 was a plano-concave mirror with radius of curvature of 100 mm. The flat mirror M with high transmission for pump light and high reflective for resonant wavelength was used as the 45° dichroic mirror.
4. Experimental results
In this experiment we use the Coherent PM30 power meter to record the output powers. Firstly, at heatsink temperature of 15 °C and physical cavity length of 33 mm, with output transmittances of 7.6%, 11.4%, 17%, 30% and 50%, we have investigated the output performance of CW Ho:GTO laser, as shown in Fig. 5. The pump thresholds were 2.02 W, 2.13 W, 2.32 W, 2.79 W and 3.83 W for above five output transmittances. In the case of output transmittance of 7.6%, the Ho:GTO laser yielded the 7.1 W output power with absorbed pump power of 18.3 W, corresponding to a slope efficiency of 43.5% respect to the absorbed pump power. When output transmittance increased to 11.4% and 17%, the output power increased to 8.6 W and 9.7 W, corresponding to a slope efficiency of 55.1% and 61.9%, respectively. With output transmittance of 30%, the Ho:GTO laser delivered 11.2 W maximum output power and 72.9% slope efficiency. In addition, 1.8% power stability was measured over a period of one hour. With the absorbed pump power of 18.3 W, the output power slowly fluctuated between 11.1 W and 11.3 W. At output transmittance of 50%, output power of 8.2 W and slope efficiency of 53.0% was achieved. A Glan-Taylor prism was employed to measure the polarization of output beam. The Ho:GTO laser has linearly-polarized output, which was verified by a contrast ratio of approximately 20 dB. Limited by pump power, we cannot demonstrate more output power in Ho:GTO laser. But we believe that the Ho:GTO laser can produce more output power because there are no power saturation phenomenon in our experiment.Figure 6 depicts the threshold power depended on the lnR1R2, the passive loss of Ho:GTO crystal was calculated to be about 0.12 cm−1.
Next, by using best output transmittance of 30%, the output characteristics of Ho:GTO laser were investigated under different heatsink temperatures. Due to quasi-three level properties, the output power of Ho laser is influenced by operating temperature. We have recorded the output powers of Ho:GTO laser under different heatsink temperatures, as shown in Fig. 7. With cavity length of 33 mm and absorbed pump power of 10.1 W, the output power of Ho:GTO laser was changed from 5.78 W to 5.19 W when the heatsink temperature increased from 11 °C to 25 °C. The slope was linear fitted to be 46 mW/°C, which indicates that the Ho:GTO laser has good temperature stability.
Thirdly, the output spectra of CW Ho:GTO laser were recorded by an optical spectrum analyzer (Bristol 721A). Figure 8 gives the output spectra with output transmittances of 7.6%, 11.4%, 17%, 30% and 50%. The single emission peak was observed for five transmittances in this experiment. With output transmittance of 7.6%, the output central wavelength of 2081.68 nm with FWHM linewidth of 1.33 nm was observed. For output transmittances of 11.4% and 17%, the output central wavelength was 2081.26 nm and 2081.25 nm, respectively. The linewidth was like 7.6% output transmittance conditions. With increasing of transmittance to 30%, the output central wavelength blue shifts to 2068.39 nm with linewidth of 1.24 nm. When the output transmittance was 50%, the central wavelength of 2067.56 nm and linewdith of 1.16 nm was recorded. With increasing of transmittance, the output central wavelength blue shifts from 2081.26 nm to 2067.56 nm. This change may can be explained by the gain cross sections of Ho:GTO crystal at room temperature. The gain cross sections can be calculated by Eq. (4) .23]. With different value of β, the gain cross sections of Ho:GTO crystal were calculated for E//a, E//b and E//c, as shown in Fig. 9. In can be seen that the preferred lasing wavelength of Ho:GTO crystal is fixted at around 2070 nm for E//c, but they are varying for E//a and E//b. With β of 0.3, the preferred lasing wavelength of Ho:GTO crystal is located at around 2086nm and 2083 nm for E//a and E//b, respectively. In the case of β of 0.5, the preferred lasing wavelength slightly shifts to 2083 nm for E//a, but it decreases to 2070 nm for E//b. When the β is 0.7, the preferred lasing wavelength shifts to 2071 nm and 2070 nm for E//a and E//b, respectively. Theoretically, the value of β increases with increasing of output transmittance . Therefore, in this work the output wavelength of about 2081 nm was achieved with lower output transmittances. With increasing of output transmittance, the output wavelength jumped to be about 2068 nm.
Finally, the M2 factors of CW Ho:GTO laser with output transmittance of 30% were measured at output power of 2 W, 5 W and 11 W. A lens with focal length of 150 mm was used to transform the Ho laser beam. The distance between lens and output coupler M2 was 250 mm. We use 90/10 knife-edge to measure the beam radii at different positions, as shown in Fig. 10. The M2 factors of Ho:GTO laser were calculated to be 1.1, 1.2 and 1.4 for output power of 2 W, 5 W and 11W, respectively. In addition, we used a camera (Cinogy CR 200HP) to take the far-field beam profile at output power of 11 W, which was shown as the inset of Fig. 10, indicating TEM00 beam propagation.
In summary, the polarized absorption and emission cross sections of Ho:GTO crystal were investigated at room temperature. A Ho:GTO crystal with dopant concentration of 1.0 at. % was employed to discover its laser performance. Under 1940.3-nm Tm fiber pumping, the maximum output power of 11.2 W at 2068.39 nm and the slope efficiency of 72.9% was achieved in the Ho:GTO laser. These results indicate that the Ho:GTO laser is a promising candidate for highly efficient 2.1-µm lasers.
National Natural Science Foundation of China (NSFC) (51572053, 51802307, 61805209 and 61775053)
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